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Wednesday, January 09, 2008

The amino acid which serves as a carrier of ammonia from skeletal muscle to liver is Alanine

038.The amino acid which serves as a carrier of ammonia from skeletal muscle to liver is:

1.Alanine

2.Methionine

3.Arginine

4.Glutamine

Answer

1.Alanine

Reference:

Lippincott 3rd Edition Page 248

Chaterjee 6th Edition Page 416

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Discussion

Two mechanisms are available in humans for the transport of ammonia from the peripheral tissues to the liver for its ultimate conversion to urea.

ÄThe first found in most tissues, uses glutamine synthetase to combine ammonia with glutamate to form glutamine – a non toxic transport form of ammonia. This glutamine is transported in the blood to liver where it is cleaved by glutaminase to produce free ammonia and glutamate.

ÄThe second transport mechanism, used primarily by muscle, involves transamination of pyruvate (the end product of aerobic glycolysis) to form alanine. Alanine is transported by the blood to the liver, where it is converted to pyruvate, again by transamination. In the liver, the pathway of gluconeogenesis can use the pyruvate to synthesize glucose, which can enter the blood and be used by muscle – a pathway called the Glucose-Alanine Cycle

Explanation

1.Alanineserves as a carrier of ammonia from skeletal muscle to liver

2.Methionine (Met, M. C5H11NO2S) is an essential nonpolar amino acid, and a lipotropic.

Methionine and cysteine are the only sulfur-containing proteinogenic amino acids. The methionine derivative S-adenosyl methionine (SAM) serves as a methyl donor. Methionine plays a role in cysteine, carnitine and taurine synthesis by the transsulfuration pathway, lecithin production, the synthesis of phosphatidylcholine and other phospholipids. Improper conversion of methionine can lead to atherosclerosis. Methionine is a chelating agent.

Tips

Methionine is one of only two amino acids encoded by a single codon (AUG) in the standard genetic code (tryptophan, encoded by UGG, is the other).